Friday, May 17, 2013

Presentation: The ML1 preamplifier with its unique design and performances was one of the best preamplifier of its time and still is despite the difficulty of repair.

Design : Probably John Curl (and also probably Tom Colangelo). Some sources state that it should have been name JC-<something>, but a dispute with Mark Levinson occured and it was renamed ML-1). Look at the JC-2 wich is designed by JC and looks very similar.

Inside view.

The ML-1 is modular. The phono modules can be replaced by the user. It has a 2 phono inputs and the classical CAMAC connectors. The power supply is external (PLS150).

From left to right on the front panel:

The input selector: Aux, Tuner, Phono1, Phono2, Tape1, Tape2

Switch: Input / Monitor

Switch: Tape 2 / Tape 1

Left channel balance: Balance L OFF /-4db / 0 / +5dB

Red led: power.

The balance.

Right channel balance: Balance R OFF /-4db / 0 / +5dB

Switch: Mono / Stereo

Switch: High / Low

Volume: 1 to 10, continuous.

There is no on/off switch as the power supply is external.

Since the preamplifier is stereo, each module exists twice, with the exception of the power supply filter DRF-3 (or DRF-4) :

D5 moving coil phono module with 54 or 60 db gain with or without CRC*.

D6 gain 60 or 66 dB.

The ULD line amplifier has a gain either of 21dB (High switch position) or 11dB (Low switch position).

*CRC: Cartridge Resonance Compensation. The CRC circuit supplied rolls the high frequencyresponse at 6dB per octave, -6dB at 20kHz. In particular, this curve has been successful in improvingthe performance of the [Fidelity Research cardridge] FR-1Mk3. Other corrective curves are available on request. Basically it's the RIAA equalization filter.

Pictures

This is not a deep analysis of the circuit, but just enough so you can understand the overall design.

Signal path

Let's start from the "top level" schematics page :

Two paths are shown :

the path in green shows the path for phono level inputs (e.g. switch is on either the Phone 1 or Phono 2 position).

the path in red in case you've selected a high level input (Aux, Tuner, Tape).

Obviously since the input selector switch works for both channels at the same time this configuration cannot happen, it's shown on the same diagram for simplicity.

The path ① in green shows that the signal from the input connector is directly fed to the box labeled Phono section. The output of this box is then routed to the same path used for the high level signals. First is goes back through the input selector ② and sent to the volume potentiometer ③. From there it traverses the balance switch ④ to reach the output ⑤ through the ULD preamplifier module.

The path in red works in a similar fashion but is more direct since it doesn't go through the phono section, obviously: from the input ① it goes to the volume potentiometer ② traversing the input selector switch (the L shaped red line). Like above it goes through the balance ③ to enter the ULD preamplifier to reach the output ④.

Phono module description

Several phono modules are described in the schematics.

The passive networks labeled ① and ② can respectively change the input impedance and the gain of the the PP module (at ③).

The operational amplifier PP contains a bunch of components, including 17 transistors (JFET's and bipolars). Its circuit diagram can be found in the schematics.

The D5 system is a bit more complex: a pre-preamplifier module named JC-1-SM is inserted before the PP op-amp. It allows the gain to be about twice as much as the above design (from 30 to 60dB).

If you look at a ML-1 with the D6 phono system from the top, here is the phono modules layout:

① is a PP-3 or 4 module.
② is the impedance/gain module, you can see the switches.
③ is the JC-1SM module.